Apec is a trade focused summit but it does not confine itself to trade.
The declaration referred to the terrorist attacks in Paris last weekend in which 129 people were killed, to bombings in Beirut the day before in which 37 were killed and the downing of a Russian airliner over Sinai on October 31 in which 224 people were killed.
"Under the shadow cast by the terrorist attacks in Paris, Beirut, and against Russian aircraft over the Sinai, and elsewhere, we strongly condemn all acts, methods, and practices of terrorism in all their forms and manifestations," the declaration said.
"We will not allow terrorism to threaten the fundamental values that underpin our free and open economies.
"Economic growth, prosperity, and opportunity are among the most powerful tools to address the root causes of terrorism and radicalisation.
"We stress the urgent need for increased international co-operation and solidarity in the fight against terrorism."
Mr Key said he spoke about terrorism particularly from an economic perspective.
"It is hard to see that it does anything than plays out very badly for tourism in Europe, for instance."
Mr Key said he had a long talk with US President Barack Obama on Wednesday about the fight against ISIS and the Paris attacks.
They walked from one meeting to another and talked for about 15 minutes.
"He was making the point that they are a terrorist group they are going to have to deal with in a certain kind of way.
"It is consistent with his public messaging which is you have just got to make sure you don't create bigger long term issues.
"He basically gave me a bit of a download on all the things they are seeing, what they are hearing, what information they have."
Mr Key also said he a good conversation with Russian Prime Minister Dmitry Medvedev who told him Russia thought there were 5000 foreign fighters fighting with Isis in Syria.